Copending patent application entitled A Communication System Network, filing date Apr. 16, 1990, Ser. No. 07/509,915 discloses a communication system network. The communication system network comprises a plurality of communication systems operably coupled to a processing multiplexer. The processing multiplexer comprises a plurality of communication system interface modules which comprise receiving means, transmitting means, destination data base circuitry, addressing circuitry, processing circuitry and signal data base circuitry. The signal data base circuitry comprises a dual port data base which stores digital signals produced by the plurality of communication systems every frame cycle. In addition, the signal data base circuitry routes stored signals to the processing circuit during the same frame cycle. The processing circuit, based on system configuration data, stored in the destination data base, indicates to the processing circuit which signals stored in the signal data base are to be processed. One of the processes performed by the processing circuit is summing together some of the audio signals to be routed to a final destination. For a complete discussion of signal processing by the processing circuit refer to the above sited co-pending patent application.
The signals stored in the signal data base are typically digital representations of audio signals. Per Mu law technology, these digitized audio signals are non-linear which presents a problem when audio signals are to be summed together. To overcome this problem, the non-linear Mu law digitized audio signals are converted into linear signals by the processing circuit. Once the processing circuit has linearized the signals, it takes their two's complement and sums them together. Once a resulting sum is achieved, it is converted back into a non-linear Mu law digital signal. This non-linear Mu law digital signal is then routed to its final destination which could be any repeater, base station, and/or speaker of a console in any of the plurality of communication systems.
A problem arises with the above described summing process when one of the digital signals is 7F (hexadecimal). When 7F is passed through the processing circuit, it is converted to 80 (hexadecimal). In Mu law 7F represents minus 0 while 80 represents full scale which will distort the resulting summation. One known solution for summing digitized audio signals is to convert 7F to FF (hexadecimal) which represents plus 0 in Mu law. By performing this conversion before the non-linear signals are converted in to linear signals, the resulting summation will be accurate. However, from to time to time, the signal data base may store digital data. If the digital data is 7F, it will be distored either by the summing process or the conversion to FF. Therefore, a need exists for a method which improves the fidelity of the summing process for predetermined digital signals.